Following is an image shot at ISO 25,000 with the NEX-5N. I took this RAW, opened in LR4 to adjust levels and exported. I did not apply noise reduction.

I know, it's nothing artistic but I tried something new (to me)

I did a bit of experimenting and found that raising the ISO does not really raise the image noise--if you lower the exposure in PP. With that said, you should use the highest ISO possible without blowing the highlights.

According to an article on Luminous Landscape on "Exposing to the Right," shooting like this gives you better color information because up to half of the camera's bit depth is used in the top 1/5 of the histogram. If that is so, using higher ISO should, in theory, give you a better signal/noise ratio in certain situations than a lower ISO. They did not talk about using this method with high ISO, so I tried it myself.

Would anyone care to try this? Take a low-light picture using your usual ISO, shutter, and aperture (raw, of course). Then take another picture, leaving the shutter and aperture where it is and increasing the ISO so that you almost blow the highlights. Finally, open them in LR or equivalent and make the exposure equal.

Note that the in-camera histogram is based on jpeg and the raw has quite a bit of headroom over the jpeg (ie. the histogram will sometimes show clipping when the raw is a full stop away from clipping). To get the most out of the raw, set the camera to portrait and set both contrast and saturation to -3. You could also try to intentionally clip some highlights and see how the raw compares.